nix3-eval
TLDR
Evaluate Nix expression
$ nix eval --expr '1 + 2'
Evaluate flake attribute$ nix eval [.#packages.x86_64-linux.default]
Evaluate nixpkgs attribute$ nix eval nixpkgs#hello.version
Output as JSON$ nix eval --json nixpkgs#hello.meta
Raw string output$ nix eval --raw nixpkgs#hello.name
Read from file$ nix eval -f [file.nix]
SYNOPSIS
nix eval [options] installable
DESCRIPTION
nix eval is the new Nix 3.x command for evaluating Nix expressions. It replaces nix-instantiate --eval with a cleaner interface and flake support.
The command evaluates expressions and prints the result.
PARAMETERS
--expr expr
Evaluate expression.--json
Output as JSON.--raw
Raw output (no quotes).-f file
Evaluate file.--apply expr
Apply function to result.--impure
Allow impure evaluation.
EXAMPLES
$ # Simple expression
nix eval --expr 'builtins.length [1 2 3]'
# Output: 3
# Flake attribute
nix eval .#packages.x86_64-linux.hello.version
# With function application
nix eval nixpkgs#lib --apply 'lib: lib.version'
nix eval --expr 'builtins.length [1 2 3]'
# Output: 3
# Flake attribute
nix eval .#packages.x86_64-linux.hello.version
# With function application
nix eval nixpkgs#lib --apply 'lib: lib.version'
CAVEATS
Part of experimental Nix 3.x commands. Flakes must be enabled. Different from nix-instantiate syntax.
HISTORY
nix eval was introduced as part of the Nix 2.4 flakes experimental features, redesigning the Nix CLI for better ergonomics.
SEE ALSO
nix(1), nix-instantiate(1), nix-build(1)
